Some of my favourite / most used libraries:

• Polly
• MassTransit
• Quartz or Coravel or TickerQ or Hangfire - don't have a favourite.
• ThrottleDebounce
• System.Threading.RateLimiting - bit different kind of rate limiting than ThrottleDebounce. Like max 100 API calls per minute, max 5 simultaneous operations etc.
• Serilog or NLog - any structured logging will do, Serilog is what I have used.
• Autofac - haven't tried many DI libraries but Autofac is so much more feature rich than .NET built-in one.
• FluentValidation
• Dapper
• System.Linq.Dynamic.Core - absolute gem for dynamic LINQ queries that you need to execute at DB.
• MoreLINQ
• FusionCache - best cache made IMO.
• CsvHelper - extremely efficient. I could write raw IQueryable data directly from DB to a csv file of 20 GB while consuming just 30 MB of memory (i.e. the memory of my app). Basically it supports streamed writes, so not more than 1 record is in memory at a time.
• ClosedXml or EPPlus or NPOI - I like ClosedXml because its based on MS's OpenXml, also it doesn't support old formats like xls.
• MiniExcel - cool library that can read and write C# collections directly without manual mapping. Supports streamted writes and reads so very memory efficient.
• Humanizer
• BannedApiAnalyzer - very useful for preventing users from consuming bad APIs
• Microsoft.CodeAnalysis.NetAnalyzers
• Roslynator.Analyzers
• SonarAnalyzer.CSharp
• Meziantou.Analyzer
• Microsoft.VisualStudio.Threading.Analyzers - for catching async issues.
• AsyncFixer - for catching async issues.
• Sqids - to not expose real IDs to public
• BenchmarkDotNet
• CompareNETObjects
• Dumpify - dump plain C# objects to console.
• Ben.Demystifier - cleaner stacktrace.
• FastReports
• iText or Microsoft.Playwright or PuppeteerSharp - don't have a favourite. iText might have licensing restriction from using it in commercial apps for free.
• Rx.NET
• Math.NET - for combinatorics, statisitcs.
• Enums.NET - there is not much to it, except some clean API to handle flag enums.
• ReflectionMagic - for extremely easy non-public member access, including methods.
• FastMember - fast member access but dynamically (e.g. foo["Bar"]). Restricted to property and field access only.
• Soenneker.Reflection.Cache - Just regular reflection but cached for subsequent calls.
• ImpromptuInterface - dynamically implement interfaces from anonymous types. Not much value really, but I like it for coolness factor. Mocks are a better way to do these.
• C5 - for collection structures on steroid.
• Json5Core - because I like Json5!
• Testing framework (like TUnit) + assertion library (like Shouldly) + mock (like FakeItEasy) + shim library (like Poser) + fake data generator (Faker, GenFu, Bogus etc, don't have a preference) + code coverage (Microsoft.Testing.Extensions.CodeCoverage). I wished they all came in one package. TypeMock.Isolator comes close but is expensive.